The present invention relates to a method and an apparatus for gasification of combustible material.
A frequent problem encountered with the harvesting, re-fining or processing of organic matter is the accumulation of waste by-products. In particular, in forestry, the harvesting and primary and secondary processing of cellulose material results in the accumulation of large quantities of biomass such as slash, twigs, branches, bark, sawdust, trimmings and scrap. In agriculture, each crop cycle and primary processing leaves substantial biomass such as bagasse, corn cobs and rice hulls that cannot be otherwise utilized. The cost of disposing of such bio-mass, the environmental damage in disposing of such materials and the waste and lost value incurred in the failure to productively utilize such materials all constitute substantial problems. The historical practice of landfilling or open-incinerator burning such organic waste is unpopular for environmental reasons and in many instances contrary to present laws and regulations. Current environmental standards in many countries preclude the use of any burners except sophisticated incinerators to bum waste.
Single purpose incinerators are viewed as inefficient and wasteful of resources. Consequently, considerable activity has been directed at developing systems, procedures and apparatus to either clean burn organic waste materials or, preferably, to convert organic waste materials into a gas, as an alterative energy source, that can be used for other purposes. One approach is shown in U.S. Pat. No. 5,666,890. The need to create a portable gasification system is also stated in U.S. Pat. No. 4,530,702 although the problem is not addressed within that art.
Many existing gasification systems require drying of the biomass in order to reduce water content (Sawyer et al; German patent DE3505329; Frohlich and Kleineindam). It is preferable for gasification equipment to be able to process wood wastes having a high range of moisture contents (e.g., 15-60%) since this is the way it is found in its natural state. Several systems, such as that described in U.S. Pat. No. 4,530,702 require operation with pellets or chips, where the biomass is pre-manufactured for combustion. Other systems, such as that described in U.S. Pat. No. 5,666,890 require a basic pre-processing of the biomass through particle size reduction in order to achieve a satisfactory conversion process. In addition to adding to the expense and the complexity of the system, these steps or requirements are often impractical for the efficient disposal of waste. It is preferred to operate a system that accepts and operates efficiently with biomass in its natural form regardless of the variance in water content without pre-processing or other additional preparation steps.
Gasification devices have been described in prior art which are suitable for individual mill or plantation operations. For example, U.S. Pat. No. 5,226,927 discloses a vertical axis, updraft reactor in which the partial oxidation of wood material is used to heat the remaining wood to a temperature of 2700 degrees Fahrenheit to produce synthesis gasxe2x80x94a mixture of carbon monoxide, hydrogen and methane. Similarly, U.S. Pat. No. 4,764,185 teaches the utility of a similar device in which the gases are moved through the system by fan or blower. In U.S. Pat. No. 4,309,195 it is disclosed that the producer gas (essentially the same as synthesis gas) formed from solid organic fuel in a gasifier apparatus is lead from the cooler/cleaner using a blower, and it is suggested that it can be used directly as a substitute for natural gas or used as a fuel for diesel or gasoline engines. A French patent (FR 2497819) discloses a gas generator which can burn damp wood or maize cobs to produce gas for use in diesel or petrol engines. Likewise, a German patent (DE 3505429) discloses a method of converting dried (15-20% moisture content) chopped wood into gas which, after cooling and scrubbing, is fed to a gas engine coupled to a generator.
In addition to the patents referred to above, there are several publications that describe devices for generating gases from cellulose waste, and for fueling an internal combustion engine which powers a generator.
Various problems are associated with all of the existing cellulose pyrolysis devices, with particular problems characteristic or specific designs. Other gasification equipment requires intricate mechanical devices to prevent bridging of the input material (e.g., U.S. Pat. No. 5,226,927; Rundstrom) but such devices consume energy, require maintenance and are not necessarily effective with all types of feedstock, for example stringy bark.
In some types of gasification equipment, the partial pyrolysis of cellulose or other hydrocarbon material results in the formation of breakdown products which are gaseous at the elevated temperature in or near the gasification zone but which condense in pipes, valves and chambers at lower temperatures, for example, at ambient temperature. Such complications occur with gasification equipment, for example, which operates in an updraft mode, that is in which pyrolysis product gases are removed from the top of the vessel.
The nominal mineral (ash) content of wood cellulose is in the 1 to 2% range but there is, in addition, the probability of the inadvertent inclusion of foreign materials, e.g., stones, nails owing to the conventional methods of handling large quantities of waste materials. Equipment used in the gasification of cellulosic waste must be able to handle such mineral contaminants with provision for removal from the pyrolysis enclosure and quenching to ambient temperature. U.S. Pat. No. 5,226,927 describes an elaborate movable, (reciprocating) grate device which could be rendered inoperative with certain sizes of inorganic materials and which, in any event, does not provide for the quenching of the ash.
It is necessary to ensure the continuous flow of gasesxe2x80x94air into the gasifier and a mixture of fuel gases, combustion products and nitrogenxe2x80x94down through the gasifier and on into adjacent pipes and/or chambers. Much of the prior art does not address this issue at all. In some cases, however, the use of a motor driven fan at the gasifier outlet is specified (e.g., U.S. Pat. No. 4,764,185 Mayer; U.S. Pat. No. 4,309,195 Rotter). In addition, to the extra costs of operating and maintaining such a blower, it can result in the removal by suction of excessive amounts of fine particles (ash, carbon) from the gasification chamber causing serious contamination problems downstream.
Accordingly, it is an object of the present invention to provide a method and apparatus for converting organic waste to usable fuel gas which is portable, which uses fewer mechanical parts and less maintenance demands than existing systems, which operates in an environmentally secure manner and which is self sustaining and, after startup, fuels itself.
It is a further object of the present invention to provide a method and apparatus that efficiently converts biomass to usable gas without the need to dry or pre-process the feedstock and which utilizes the water inherent in most biomass as part of the pyrolysis procedure regardless of the variance in quantity.
It is an additional object of the present invention to provide an efficient means by which ash is removed from the burn chamber without interruption in the bum/pyrolysis processes, and an efficient means by which the synthesis gas is removed from the gasifier without mechanical moving parts such as a blower or a fan.
Therefore, according to a first aspect of the invention, there is provided a gasifier comprising a feed hopper, a burn chamber disposed to receive feed material from the feed hopper, a gas supply operably connected to the burn chamber for supplying gas containing oxygen to the burn chamber, a water supply operably connected to the burn chamber for supplying water to the burn chamber and a burn chamber outlet conduit in the burn chamber for egress of gases produced within the burn chamber by reaction of pyrolysis products according to the water gas.
According to a further aspect of the invention, an evacuator, for example an internal combustion engine, has an intake operably connected to the burn chamber outlet conduit for drawing gas along a flow path from the burn chamber into the evacuator.
According to a further aspect of the invention, particulate removal apparatus is provided in the flow path between the intake and burn chamber outlet conduit. Preferably, the particulate removal apparatus is selected from the group consisting of scrubbers and filters.
According to a further aspect of the invention, the burn chamber is formed of an upper chamber and a lower chamber below the upper chamber, the upper chamber is separated from the lower chamber by a hinged plate; and the hinged plate is operable upon hinging to transfer feed material under force of gravity from the upper chamber into the lower chamber.
According to a further aspect of the invention, flow of gas towards the evacuator defines a downstream direction, and the burn chamber is defined by an encircling wall, the gasifier further comprising a grate within the burn chamber situated downstream from gas supply and the water supply, the grate comprising plates forming a support for a coal bed during operation of the gasifier.
According to a further aspect of the invention, reciprocating angled plates are interleaved with the plates of the grate, the reciprocating angled plates being arranged to reciprocate parallel to the downstream direction and cause debris on the grate to move towards the encircling wall.
According to a further aspect of the invention, there are provided ports in the encircling wall adjacent the grate for the removal of debris from the burn chamber.
According to a further aspect of the invention, the water supply is a source of steam, which may be a coiled pipe encircling the burn chamber.
According to a further aspect of the invention, the gas supply is connected to a source of heated air.
According to a further aspect of the invention, the egress of gases from the burn chamber follows a flow path passing above a water reservoir. Preferably, the burn chamber is bounded on one side by water in the water reservoir. Preferably, the burn chamber outlet conduit comprises a pipe having an opening for entry of gas into the pipe, the opening being on a side of the pipe that faces the water in the water reservoir.
According to a further aspect of the invention, there is provided a method of gasifying feed material by feeding feed material into a burn chamber, burning the feed material in the burn chamber in the presence of water to generate sufficient heat to pyrolyze the feed material and produce gas by the water gas reaction; and drawing the gas from the burn chamber.
Drawing of the gas from the burn chamber may be carried out by operation of an internal combustion engine, the intake of which is connected to the burn chamber.
Drawing gas from the burn chamber may be carried out by passing the gas through a particular removal apparatus, which preferably is formed by a scrubber followed by a dry filter. Preferably, the gas is drawn over a water reservoir. Material flow into the burn chamber is through a feed material inlet and an air supply.
Preferably, the burn chamber comprises a grate, and the temperature at the grate is maintained at a temperature of about 1800 degrees F. to 2200 degrees F. by controlled injection of water and oxygenated gas.
These and other features of the invention are described in the detailed description of the invention and claimed in the claims that follow.